FIELD: This invention relates to the field of land geophysical seismic investigation, and specifically to combustible gas seismic source apparatuses and methods of sensing and recording near-surface reflected seismic energy.
Near-surface geophysical seismic investigations are differentiated from shallow seismic investigations in that near surface investigations are most often directed to the geology of the near surface weathered layer. Shallow seismic investigations are directed to gathering information about layers below the weathered layer. Near-surface seismic investigations are conducted with a light weight source suited to hand-portable operations in either urban or rural settings. These apparatuses rely on the repeatability of low power energy sources which allow for a summing of reflected traces which enhance the signal to noise ratio of the traces.
The problems associated with near-surface seismic investigations also differ from those of shallow seismic investigations in that the weathering layer is often comprised of low velocity material. This low velocity material stretches out the returning reflected wave in time where it is subjected to destructive interference from groundroll. Groundroll is comprised of high-amplitude, low frequency energy which is the result of elastic rebound from the impact of a pressure pulse on or near the earth's surface. Groundroll energy is generated by source apparatuses currently being used in near-surface seismic investigations. These types of apparatuses include vibrating masses, pressurized gases in combination with liquids, combustible gas mixtures, sledge hammers, weight drops, shotguns and modified rifles.
Various gas exploding pulse sources have been developed and successfully employed in both shallow and deep seismic investigations. Most notable are U.S. Pat. Nos. 3,235,027 of L. G. Kilmer, 3,361,226 of S. E. Szasz and 4,043,420 of R. J. L. Zena and W. W. Horsman. These apparatuses are powered by the combustion of pre-mixed gases within an enclosed combustion housing which is connected to a rigid bottom reaction plate. The expansion of gases, during combustion, imparts a downward force on the rigid reaction plate which is pressed against the ground. The pulse of energy caused by the movement of the plate initiates the seismic wave.
These prior art energy pulse sources powered by explosions do not address the destructive interference of groundroll. The use of a reactive plate to initiate a seismic wave energy increases the amplitude and duration of the groundroll. Signals sensed from groundroll interfere with signals of interest which are produced while the groundroll is occurring, not afterward.
One approach to the generation and earth coupling of seismic waves is to use an energy focusing device. One such device, shown in U.S. Pat. No. 3,275,098 of W. S. Filler, requires a firing chamber which opens into a shock tube. The shock wave, which is generated by an explosive device in the firing chamber, is directed to the earth's surface through a tubular or conical shock tube.
A major limitation of the above described apparatus and its inherent method in use for near-surface seismic investigations is that if the shock tube is conical portions of the energy are reflected back to the apex of the cone and back to the ground, causing additional waves to strike the surface and increase the duration of the destructive interference of groundroll.
Another serious limitation of the method and apparatus exemplified by Filler is that relatively low energy but highly expedient explosives such as combustible gas mixtures do not produce sufficiently high velocity differential pressure ridges; i.e. shock waves.
Accordingly, the prime objective of the subject invention is to provide seismic source apparatus, the operation of which is relatively free of the influences of groundroll phenomena. A second objective is that satisfactorily high energy shock waves can be produced using relatively low energy explosives. A third objective is to limit the amount of recoil of the apparatus relative to the power of the explosions.